Cascode active shunt gate oxide protect during electrostatic discharge event

ABSTRACT

A method and apparatus to provide electrostatic discharge (ESD) protection to electronic circuits using a gate clamp circuit.

REFERENCE TO RELATED APPLICATION

This application is a continuation of Ser. No. 11/339,053 filed Jan. 24,2006, which is hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to the field of electrostatic discharge (ESD)circuits and, in particular, to ESD shunt circuits.

BACKGROUND

Electrostatic discharge (ESD), which is the rapid discharge of staticelectricity from one conductive material to another, can damage computerequipment. An electric charge transfers from one conductor to anotherbecause of a difference in electrical potential of the conductivebodies. As used herein, an ESD event is the occurrence of anelectrostatic discharge.

ESD can negatively affect computer equipment in many ways. An ESD eventduring manufacturing can cause product defects. An ESD event duringoperation can cause a product to malfunction or incur temporary orpermanent damage. To prevent or control ESD events during operation,conventional computer equipment often includes ESD circuitry to routedischarged static electricity away from critical components (e.g. to aground reference). In general, ESD circuitry may be implemented toprotect integrated circuits and microchips. In a particular example, ESDcircuitry may be used to protect a random access memory (RAM) device.

FIG. 1 depicts a conventional ESD circuit. The conventional ESD circuitincludes a trigger circuit and an ESD shunt circuit. The trigger circuitcontrols the operation of the ESD shunt circuit. When the triggercircuit detects an ESD current on the power source, vpwr2, (which isalso present on the power source, vpwr1), the trigger circuit sends acontrol signal, trig, to the ESD shunt circuit. The ESD shunt circuitincludes a high voltage (HV) transistor. When the ESD shunt circuit isturned on by the trigger circuit, the ESD shunt circuit provides anelectrical path between the power source, vpwr1, and the groundreference, vgnd, so that the ESD voltage and current are conducted tothe ground reference. In this way, electrical components connected tothe power source, vpwr1, are protected from damage due to the ESD event.The high voltage transistor is protected against damage during the ESDevent because it has a relatively thick gate oxide layer. An exemplarythickness of a thick gate oxide layer of a high voltage transistor isapproximately 60 Å (60×10⁻¹⁰ m).

During normal operation, before and after an ESD event, the controlsignal is driven low (e.g., to vgnd) so that the high voltage transistoris turned off. Turning off the high voltage transistor preventsunintended current flow from the voltage source to the ground referenceduring normal operation. In order for the low control signal to turn offthe high voltage transistor, the high voltage transistor must have apositive threshold voltage. However, many modern technologies use nativetransistors that are not doped to raise the threshold voltage. In fact,some modern technologies use threshold voltages that are zero orslightly negative for cost savings in manufacturing. In addition toincreasing the mask and manufacturing cost, the high voltage transistorsyield lower drive current per unit width of the transistor compared to alow voltage transistor under the same bias conditions.

Currently, high voltage transistors with a positive threshold voltageare not used as frequently in many newly developed products andtechnologies. Additionally, new technologies often incorporate lowvoltage (LV) transistors in order to reduce power requirements or obtainother benefits. However, the use of low voltage transistors can presenta challenge in designing ESD circuits because low voltage transistors donot sustain ESD voltages to the same degree as high voltage transistors.For example, a low voltage transistor may be designed for normaloperation at approximately 1.8 volts, but a high voltage transistor maybe designed for normal operation at greater than 1.8 volts. The relativeoperating voltages of low and high voltage transistors is related to therelative gate oxide thickness of low and high voltagetransistors—transistors with thicker gate oxide layers can operate athigh voltages. An exemplary thickness of a gate oxide layer of a lowvoltage transistor is approximately 20 Å (20×10⁻¹⁰ m).

Thus, where a single high voltage transistor is used to provide ESDprotection in an ESD circuit, the ESD shunt circuit may be incompatiblewith a modern technology that uses low voltage (LV) transistors and/orlower threshold voltage transistors. Additionally, the protectivefunctionality of the ESD shunt circuit is limited by the gate oxidestress voltage of the single high voltage transistor. Addition of a highvoltage transistor with non-zero, positive threshold voltage to theprocess increases the mask and manufacturing cost. For high volume, lowcost products this is not an economically viable solution.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings.

FIG. 1 depicts a conventional electrostatic discharge (ESD) circuit.

FIG. 2 depicts one embodiment of an ESD circuit having a modified ESDshunt circuit.

FIG. 3 depicts another embodiment of an ESD circuit having a modifiedESD shunt circuit.

FIG. 4 depicts one embodiment of a gate clamp circuit.

FIG. 5 depicts one embodiment of operating waveforms of an ESD circuitduring normal operation.

FIG. 6 depicts one embodiment of operating waveforms of an ESD circuitduring an ESD event.

FIG. 7 depicts one embodiment of an ESD shunt method.

FIG. 8 depicts one embodiment of a static random access memory (SRAM)with a global trigger circuit and a plurality of local ESD shuntcircuits.

DETAILED DESCRIPTION

The following description sets forth numerous specific details such asexamples of specific systems, components, methods, and so forth, inorder to provide a good understanding of several embodiments of thepresent invention. It will be apparent to one skilled in the art,however, that at least some embodiments of the present invention may bepracticed without these specific details. In other instances, well-knowncomponents or methods are not described in detail or are presented insimple block diagram format in order to avoid unnecessarily obscuringthe present invention. Thus, the specific details set forth are merelyexemplary. Particular implementations may vary from these exemplarydetails and still be contemplated to be within the spirit and scope ofthe present invention.

Embodiments of the present invention include various operations, whichwill be described below. These operations may be performed by hardwarecomponents, software, firmware, or a combination thereof. As usedherein, the term “coupled to” may mean coupled directly or indirectlythrough one or more intervening components. Any of the signals providedover various buses described herein may be time multiplexed with othersignals and provided over one or more common buses. Additionally, theinterconnection between circuit components or blocks may be shown asbuses or as single signal lines. Each of the buses may alternatively beone or more single signal lines and each of the single signal lines mayalternatively be buses.

Certain embodiments may be implemented as a computer program productthat may include instructions stored on a machine-readable medium. Theseinstructions may be used to program a general-purpose or special-purposeprocessor to perform the described operations. A machine-readable mediumincludes any mechanism for storing or transmitting information in a form(e.g., software, processing application) readable by a machine (e.g., acomputer). The machine-readable medium may include, but is not limitedto, magnetic storage medium (e.g., floppy diskette); optical storagemedium (e.g., CD-ROM); magneto-optical storage medium; read-only memory(ROM); random-access memory (RAM); erasable programmable memory (e.g.,EPROM and EEPROM); flash memory; electrical, optical, acoustical, orother form of propagated signal (e.g., carrier waves, infrared signals,digital signals, etc.); or another type of medium suitable for storingelectronic instructions.

FIG. 2 depicts one embodiment of an ESD circuit 100 having a modifiedESD shunt circuit 110. The illustrated ESD circuit 100 also includes atrigger circuit 120 and an implicit resistor 130. The resistive element130 is designated as Rvgnd. Although the trigger circuit 120, ESD shuntcircuit 110, and resistive load 130 are schematically shown connected toone another, the actual interconnections among the trigger circuit 120,the ESD shunt circuit 110, and the resistive load 130 may be routed inmultiple manners, including indirect coupling via other components notshown in FIG. 2.

The trigger circuit 120 includes a power input, vpwr2, and two biasvoltage inputs, vcascp and vcascn. The power input is tied to otherelectronic components (not shown) which are to be protected by the ESDcircuit 100. The trigger circuit 120 detects an ESD current or voltageon the power input line. The bias voltage inputs provide bias voltagesfor gate oxide overstress protection during normal (i.e., non-ESD)operation of the ESD circuit 100. Exemplary values for the bias voltageinput signals are 3.6V for vcascp and 2.5V for vcascn. These input biasvoltages are generally a function of the chip supply voltages vpwr2 andvpwr1. The trigger circuit 120 also includes two output lines, scasc andstrig, which are coupled to the ESD shunt circuit 110 to provide controlsignals for the ESD shunt circuit 110. These two output control signalswill be described in further detail below and with reference to FIGS. 5and 6. The trigger circuit 120 is also coupled to a ground reference,vgnd trig.

The illustrated ESD shunt circuit 110 includes a low voltage (LV)transistor 140 and a first high voltage (HV) transistor 150. In oneembodiment, the first high voltage transistor 150 may be a nativetransistor. The low voltage transistor 140 is designated as M1 a. Thehigh voltage transistor 150 is designated as M1 b. The low voltagetransistor 140 and the first high voltage transistor 150 are arranged toform a transistor stack coupled between a power source, vpwr1, and aground reference, vgnd_shunt. In one embodiment, the power source linesof the trigger circuit 120 and the ESD shunt circuit 110 are directly orindirectly coupled together so that an ESD event on either power sourceline may be detected by the trigger circuit 120.

In the depicted configuration, the low voltage transistor 140 acts as aswitch to turn on the transistor stack. During normal operation, beforeand after an ESD event, the low voltage transistor 140 may be turned offto prevent current from passing between the power source and groundreference via the ESD shunt circuit 110. In one embodiment, the triggercircuit 120 supplies a bias voltage to the first high voltage transistor150 to prevent overstress of the low voltage transistor 140 and thefirst high voltage transistor 150. An exemplary value of the biasvoltage input signal to the first high voltage transistor 150 is 2.5V.

During an ESD event, the trigger circuit 120 may turn on the low voltagetransistor 140 and the first high voltage transistor 150. Exemplaryvalues for the voltage input signals to turn on the low voltagetransistor 140 and the high voltage transistor 150 are 4.0V and 8.0V,respectively. The low voltage transistor 140 acts as a switch, asdescribed above. The first high voltage transistor 150 acts as aresistive load to dissipate at least some of the electrical powerresulting from the ESD event. In this manner, the ESD shunt circuit 110may operate in conjunction with the trigger circuit 120 to divert an ESDcurrent to ground without damaging any electrical components.

Although FIG. 2 depicts one embodiment of an ESD shunt circuit 110having one low voltage transistor 140 and one high voltage transistor150, other embodiments of the ESD shunt circuit 110 may includeadditional low voltage transistors 140, high voltage transistors 150, orboth. Additionally, the trigger circuit 120 may have fewer or moreoutput control lines to supply control signals to the ESD shunt circuit110. Although FIG. 2 shows one control line for each transistor in theESD shunt circuit 110, other embodiments may include fewer or morecontrol lines than the number of transistors in the ESD shunt circuit110.

FIG. 3 depicts another embodiment of an ESD circuit 200 having amodified ESD shunt circuit 210. Although some similarities may existbetween the ESD circuit 100 of FIG. 2 and the ESD circuit 200 of FIG. 3,the ESD shunt circuit 210 of FIG. 3 is different from the ESD shuntcircuit of FIG. 2. In particular, the ESD shunt circuit 210 of FIG. 3includes a second high voltage transistor 220, in addition to a firsthigh voltage transistor 150 and a low voltage transistor 140. In oneembodiment, the second high voltage transistor 220 may be a nativetransistor. The second high voltage transistor 220 is designated as M1c. The ESD shunt circuit 210 also includes a gate clamp circuit 230coupled to one of the input control lines, strig, and to the groundreference, vgnd_shunt. One example of the gate clamp circuit is shown inand described in more detail with reference to FIG. 4. Although the ESDshunt circuit 210 includes the second high voltage transistor 220 andthe gate clamp circuit 230, other embodiments of the ESD shunt circuit210 may include the second high voltage transistor 220 and not the gateclamp circuit 230. Alternatively, another embodiment of the ESD shuntcircuit 210 may include the gate clamp circuit 230 and not the secondhigh voltage transistor 220.

In the depicted configuration, the low voltage transistor 140 acts as aswitch to turn on the transistor stack. During normal operation, beforeand after an ESD event, the low voltage transistor 140 may be turned offto prevent current from passing between the power source and groundreference via the ESD shunt circuit 210. In one embodiment, the triggercircuit 120 supplies a bias voltage to the second high voltagetransistor 220 to prevent overstress of the low voltage transistor 140,the first high voltage transistor 150, and the second high voltagetransistor 220. An exemplary value of the bias voltage input signal tothe second high voltage transistor 220 is 3.6V. In one embodiment, thefirst high voltage transistor 150 may be turned on or off during normaloperation.

During an ESD event, the trigger circuit 120 may turn on the low voltagetransistor 140, the first high voltage transistor 150, and the secondhigh voltage transistor 220. Exemplary values for the voltage inputsignals to turn on the low voltage transistor 140, the first highvoltage transistor 150, and the second high voltage transistor 220 are4.0V, 8.0V, and 8.0V, respectively. The low voltage transistor 140 actsas a switch, as described above. The first high voltage transistor 150acts as a resistive load to dissipate at least some of the electricalpower resulting from the ESD event. The second high voltage transistor220 also acts as a resistive load to dissipate the electrical power fromthe ESD event. In this manner, the ESD shunt circuit 210 may operate inconjunction with the trigger circuit 120 to divert an ESD current toground without damaging any electrical components.

In the illustrated embodiment, the gate clamp circuit 230 receives abias voltage input signal, strig, from the trigger circuit. The samebias voltage output line from the trigger circuit 120 is also coupled tothe first high voltage transistor 150. However, the gate clamp circuit230 provides a voltage-clamped control signal, g1, to the low voltagetransistor 140, rather than the unclamped control signal, strig. Anexemplary value of the clamped voltage control signal, g1, is 4.0Vmaximum during an ESD event. In another embodiment, the gate clampcircuit 230 may be coupled to an independent control signal output fromthe trigger circuit 120. Alternatively, the gate clamp circuit 230 maybe coupled to another bias voltage input signal such as the bias voltageinput signal, scasc.

FIG. 4 depicts one embodiment of a gate clamp circuit 230. Theillustrated gate clamp circuit 230 includes an input line, an outputline, and a ground reference line. In one embodiment, the input line iscoupled to an output line, strig, from the trigger circuit 120, as shownin FIG. 3. In another embodiment, the input line of the gate clampcircuit 230 may be coupled to another control line of the ESD circuit210. The output line of the gate clamp circuit 230 is coupled to thegate of the low voltage transistor 140 of the ESD shunt circuit 210. Theground reference line is coupled to a ground reference source of the ESDcircuit 200.

In the depicted embodiment, the gate clamp 230 also includes a resistiveload 240, a first low voltage clamping transistor 250, and a second lowvoltage clamping transistor 260. In one embodiment, the clampingtransistors 250 and 260 are diode-connected n-channel transistors. Theresistive load 240 is designated as Rclmp and coupled between the inputand output lines of the gate clamp circuit 230. The first low voltageclamping transistor 250 is designated as Mclmp1. The second low voltageclamping transistor 260 is designated as Mclmp2. The clampingtransistors 250 and 260 are coupled in a cascode stack arrangementbetween the output line and the ground reference line. In particular,the clamping transistors 250 and 260 are connected as diodes, with thegate of each clamping transistor 250 and 260 electrically connected tothe drain of the corresponding clamping transistor 250 and 260. In thedepicted configuration, the clamping transistors 250 and 260 function asa voltage divider so that the output signal, g1, is clamped atapproximately twice the voltage drop of a single clamping transistor 250or 260. An exemplary value of the clamped voltage on the output line ofthe gate clamp circuit 230 is 4.0V maximum during an ESD event.

Although a particular schematic design for the gate clamp circuit 230 isillustrated in FIG. 4, other embodiments of the ESD circuit 200 mayutilize another type of gate clamp circuit 230. For example, the gateclamp circuit 230 may include fewer or more clamping transistors 250 and260. In another embodiment, the gate clamp circuit 230 may beimplemented with a diode-connected n-channel transistor instead of theresistive load 240. In another embodiment, the gate clamp circuit 230may be implemented with resistors instead of the low voltage clampingtransistors 250 and 260, although the resulting clamped voltage may bemore dependent on the current through the gate clamp circuit 230. Otherembodiments of the gate clamp circuit 230 also may be used to generate aclamped voltage on the output line.

Additionally, the ESD shunt circuit 210 may have other configurations.For example, the ESD shunt circuit 210 may have fewer or moretransistors. In another embodiment, the high voltage transistors 150 and220 and the low voltage transistor may be arranged in another order. Inanother embodiment, the transistors may be PMOS transistors, which mayinfluence the arrangement of the low voltage transistor 140 in relationto the high voltage transistors 150 and 220, as well as the controlsignals from the trigger circuit 120. In other embodiments, the ESDcircuit 200 may include an ESD shunt circuit 210 with othercharacteristics.

FIG. 5 depicts one embodiment of operating waveforms 300 of an ESDcircuit 200 during normal operation. In other words, the operatingwaveforms 300 of FIG. 5 correspond to a time other than during theoccurrence of an ESD event. In particular, the operating waveforms 300include the power signal, vpwr2, on the power source line, the outputbias control signal, scasc, and the output control signal, strig, fromthe trigger circuit 120, and the clamped voltage signal, g1, from thegate clamp circuit 230. The trigger circuit 120 monitors the powersource line to detect the power signal. During normal operation, thepower signal does not change rapidly, so the trigger circuit 120 detectsa slow power-up ramp on the power source line at time t0.

In one embodiment, the trigger circuit 120 is configured to generate abias control signal on one of the output control lines during normaloperation. For example, the trigger circuit 120 may generate the biascontrol signal at a time t1 in response to the slow power-up ramp on thepower source coupled to the trigger circuit 120. Driving the biascontrol signal on the output line coupled to the second high voltagetransistor 220 protects the stacked transistors 140, 150, and 220 fromovervoltage stress during normal operation. In the absence of the biascontrol signal, the second high voltage transistor 220 will continue tosee increasing potential difference across its gate oxide between thegate and drain terminals resulting in gate oxide overstress that canlead to damage of the gate oxide. The bias control signal helps tominimize the potential difference across the gate oxide therebyprotecting the second high voltage transistor 220. The bias controlsignal also acts to limit the voltage on the source of second highvoltage transistor 220, which is coupled to the drain of the first highvoltage transistor 150. This limits the maximum voltage between the gateand drain terminals of the first high voltage transistor 150, therebylimiting its gate oxide stress and protecting the first high voltagetransistor 150 from damage during normal operation. The trigger circuit120 is configured to maintain the other control signal low during normaloperation so that the resulting gate clamp signal is also low duringnormal operation. This turns off the ESD shunt circuit 210 so thatcurrent passes between the power source and ground via the ESD shuntcircuit during normal operation.

FIG. 6 depicts one embodiment of operating waveforms 350 of an ESDcircuit 200 during an ESD event. In particular, the operating waveforms350 correspond to the same signals shown in FIG. 5, including the powersignal, vpwr2, on the power source line, the output bias control signal,scasc, and the output control signal, strig, from the trigger circuit120, and the clamped voltage signal, g1, from the gate clamp circuit230. To detect the occurrence of an ESD event, the trigger circuit 120monitors the power source line to detect the power signal. When an ESDevent occurs, the trigger circuit 120 detects a rapid rise in the powersignal. The beginning of the ESD event is designated as time t0.

Upon recognizing an ESD event, the trigger circuit 120 turns on thefirst and second high voltage transistors 150 and 220 at a timedesignated as time t1. In one embodiment, the trigger circuit generatesand transmits a high pulse on the control lines coupled to the ESD shuntcircuit 110. For example, the trigger circuit 110 may output pulseshaving an amplitude approximately equal to the voltage on the powersource, vpwr2. Nearly simultaneously, the trigger circuit 120 turns onthe low voltage transistor 140 at a time designated as time t2. In oneembodiment, the trigger circuit 120 turns on the low voltage transistor140 as a result of sending the high pulse on one of the control lines towhich the gate clamp circuit 230 is coupled. In one embodiment, the timedelay between times t1 and t2 is only due to the transmission delay ofthe gate clamp circuit 230. Alternatively, the time delay between timest1 and t2 may be controlled by the trigger circuit 120 or anothercomponent in the ESD circuit 200.

The gate clamp circuit 230 clamps the control voltage signal from thetrigger circuit 120 to the low voltage transistor 140. Clamping thecontrol signal in this manner protects the low voltage transistor 140from overstress that might result from using an unclamped controlvoltage. Between the times designated as t2 and t3, the ESD shuntcircuit 210 is active and shunts the potentially harmful ESD charge tothe ground reference. Exemplary times between t0, t1, t2, and t3 are 6ns, 200 ps, 100 ns, respectively. However, the actual times may deviatedepending on the actual characteristics of the ESD event and the ESDcircuit 200.

FIG. 7 depicts one embodiment of an ESD shunt method 400. Theillustrated ESD shunt method 400 functionally shows the operations ofthe ESD circuit 200 during the approximate time of an ESD event. Inparticular, the ESD shunt method 400 begins when the trigger circuit 120recognizes 405 an ESD event. The trigger circuit 120 then generates 410the control signals, scasc and strig, for the ESD shunt circuit 210. Bytransmitting the control signals to the ESD shunt circuit 210, thecontrol signals turn on 415 the high voltage transistors 150 and 220and, nearly simultaneously, turn on 420 the low voltage transistor 140.As described above, the low voltage transistor 140 acts as a switch toturn on the ESD shunt circuit 210 in order to shunt 425 the ESD currentand voltage to ground. The trigger circuit 120 then turns off 430 theESD shunt circuit 210 so that the power source is no longer connected toground.

FIG. 8 depicts one embodiment of a static random access memory (SRAM)500 with a global trigger circuit 120 and a plurality of local ESD shuntcircuits 210. For clarity and convenience, the schematic illustration ofthe SRAM 500 omits many typical features and components of a memorydevice. Nevertheless, the depicted SRAM 500 portrays how an ESD circuit200 may be used to protect the input/output (I/O) circuits 510 of theSRAM 500. In a similar manner, an ESD circuit may be used to protectother components of the SRAM 500 or another electronic device.

In one embodiment, each of the ESD shunt circuits 210 is locatedrelatively close to one of the I/O circuits 510 in order to protect theSRAM 500 from an ESD event that occurs at the I/O circuits 510. In manyinstances, an ESD event may occur remotely from the SRAM 500, but mayarrive at the I/O circuits 510 in route to the internal components ofthe SRAM 500. In another embodiment, the ESD event may originate withinthe SRAM 500. In any case, multiple ESD shunt circuits 210 may bedistributed throughout the SRAM 500, or another electronic device, atlocations where an ESD event is likely to originate with respect to thatdevice. In this way, the each of the ESD shunt circuits 210 may belocally positioned next to or near a particular component within theSRAM 500.

The trigger circuit 120, on the other hand, is shown as a global triggercircuit. In particular, a single trigger circuit 120 supplies controlsignals to all of the ESD shunt circuits 210. Alternatively, the SRAM500 may include multiple trigger circuits 120 so that the ratio oftrigger circuits 120 to ESD shunt circuits 210 may be approximately1:10, 1:4, 1:1, or another ratio. In another embodiment, the ESD circuit200 may have a global gate clamp circuit 230, rather than individualgate clamp circuits 230 for each ESD shunt circuit 210. For example, aglobal gate clamp circuit 230 may be included in the global triggercircuit 120. In another embodiment, the global gate clamp circuit 230may be separate from the global trigger circuit 120 or there may bemultiple regional gate clamp circuits 230 distributed within the SRAM.

Certain embodiments of the method, apparatus, and system described aboveoffer advantages, compared to conventional technologies. One advantageof having a combination of low voltage and high voltage transistors isthe ability to have some transistors with zero or negative thresholdvoltages and some transistors with positive threshold voltages. Forexample, the high voltage transistors 150 and 220 may have nominallyzero threshold voltages, and the low voltage transistor 140 may have apositive threshold voltage. However, other combinations of thresholdvoltages may be implemented.

One advantage of using multiple transistors in a cascode arrangement isthe ability to operate at voltages higher than the operating capacity ofany one transistor alone. In one embodiment, using appropriate biasvoltages for the control signals, scasc and strig, facilitatesprotection of the transistors 140, 150, and 220 against gate oxideoverstress at the higher voltages.

Although the operations of the method(s) herein are shown and describedin a particular order, the order of the operations of each method may bealtered so that certain operations may be performed in an inverse orderor so that certain operation may be performed, at least in part,concurrently with other operations. In another embodiment, instructionsor sub-operations of distinct operations may be in an intermittentand/or alternating manner.

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative sense rather than arestrictive sense.

1. A system, comprising: a memory circuit, comprising: an input/output(I/O) circuit; and an electrostatic discharge (ESD) shunt circuitcoupled to the I/O circuit to shunt an ESD current to ground, the ESDshunt circuit comprising: a low voltage ESD transistor; a first highvoltage ESD transistor in series with the low voltage ESD transistor; agate clamp circuit coupled to the low voltage ESD transistor to clamp agate-to-source voltage of the low voltage ESD transistor, the gate clampcircuit having an input, a low voltage output coupled to the gate of thelow voltage ESD transistor and a ground reference coupled to the sourceof the low voltage ESD transistor, wherein the gate clamp circuit limitsthe low voltage output; and a trigger circuit coupled to the ESD shuntcircuit to trigger the ESD shunt circuit in response to an ESD event. 2.The system of claim 1, wherein the trigger circuit is configured toprovide a trigger signal to the gate clamp circuit and the first highvoltage ESD transistor.
 3. The system of claim 2, further comprising afirst power line coupled to the ESD shunt circuit and a second powerline coupled to the trigger circuit, wherein the ESD event on the firstpower line is recognizable on the second power line.
 4. The system ofclaim 2, wherein the trigger circuit further comprises a first inputline and a second input line to supply at least two distinct biasvoltages to the trigger circuit.
 5. The system of claim 2, wherein theESD shunt circuit further comprises a second high voltage ESD transistorin series with the first high voltage ESD transistor, and wherein thetrigger circuit is configured to provide a control signal to the secondhigh voltage ESD transistor.
 6. An apparatus, comprising: means forshunting an electrostatic discharge (ESD) current to ground, wherein themeans for shunting the ESD current includes a low voltage switchingmeans; and means for providing a control signal to the low voltageswitching means, comprising means for clamping the control signal tolimit stress on the low voltage switching means, wherein the means forclamping the control signal further comprises a resistive load coupledbetween an input of the means for clamping the control signal and thelow voltage switching means, and wherein the means for shunting the ESDcurrent further comprises a high voltage resistive means coupled to thelow voltage switching means.
 7. The apparatus of claim 6, wherein themeans for clamping the control signal comprises at least two low voltagetransistors coupled in series between an input of the low voltageswitching means and a ground reference.
 8. The apparatus of claim 7,further comprising means for providing at least one additional controlsignal to the high voltage resistive means.